The present invention relates generally to special reproducing systems in apparatus for reproducing rotary recording mediums. More particularly, the present invention relates to a special reproducing system in an apparatus for reproducing, by means of a reproducing transducer, a video signal from a rotary recording medium on which the video signal has been recorded along a spiral track, at the rate of two or more fields per track turn. The reproducing transducer of the system traces each track turn a number of times to accomplish a special reproduction free from any unnatural motion of the reproduced picture.
In general, in a system for recording a television video signal on a rotating recording medium, the fewer the number of fields of the video signal recorded per revolution of the rotating recording medium, the higher the required rotational speed of the rotating recording medium. When the rotational speed is high, the system is subjected to unduly severe mechanical conditions so that vibration occurs and/or automatic control becomes difficult. Moreover, the recording capacity of the recording medium is reduced.
On the other hand, the greater the number of fields of the video signal recorded per revolution of the rotating recording medium, the lower is the required rotational speed of the rotating recording medium. Thus, when a large number of fields are recorded per revolution, the system is not subjected to excessively severe mechanical conditions. Furthermore, the recording capacity of the recording medium becomes large. On the other hand, however, in the case where the signal is recorded on the recording medium as a variation in geometrical shape and is thereafter reproduced as capacitance variation or optical variation, the wavelength of the signal recorded on the recording medium becomes short so that the reproducing element becomes unable to reproduce satisfactorily.
In view of the above described circumstances, it is most desirable to record a video signal at the rate of four fields, that is, two frames, per revolution of the rotating recording medium.
In this connection, a novel "Information signal recording and reproducing system" has been described in U.S. patent application Ser. No. 785,095 (filed Apr. 6, 1977) and assigned to the assignee of the present application and now U.S. Pat. No. 4,331,976. A continuation-in-part of this application was filed as U.S. patent application Ser. No. 33,324, filed Apr. 25, 1979 and now U.S. Pat. No. 4,315,283. The system described by these two applications has already been reduced to practice. In accordance with this system, a video signal is recorded along a track of spiral form as a geometrical shape variation, without a reproducing stylus guide groove. The track is on a rotating recording medium having a flat surface. Since there is no guide groove in the rotating recording medium, the reproducing stylus can move smoothly from one track turn to another at the time of still-picture, slow-motion, or quick-motion reproduction. Accordingly, in spite of the use of a reproducing stylus in this system, special reproducing modes such as still-picture reproduction can be smoothly carried out.
However, in the case where still-picture reproduction is carried out by repeatedly reproducing the same track turn, if a number of fields (for example, four fields as mentioned above) is recorded along the same track turn, the reproduced still-picture will not be perfectly still, but will appear unnatural and indistinct. More specifically, particularly where the information content is one of rapid motion, the position of a moving object at the starting point of a given track turn will differ considerably from its position at the terminal point of the same track turn in the reproduced picture. For this reason, when the same track turn is repeatedly reproduced, each moving object repeatedly undergoes reciprocal motion between these two different positions, and the reproduced image appears to flicker or vibrate. Also at the time of slow-motion reproduction when the same track turn is reproduced a plurality of times and then the reproducing element (stylus) shifts to the succeeding track turn, the reproduced picture will move unnaturally.